1. Field of the Invention
This invention relates to a magnetic-material-dispersed resin carrier, a two-component type developer having this magnetic-material-dispersed resin carrier and a toner, and an image-forming method making use of this two-component type developer. More particularly, the present invention relates to a magnetic-material-dispersed resin carrier used together with a toner to constitute a two-component type developer for developing an electrostatic latent image by electrophotography, a two-component type developer having this magnetic-material-dispersed resin carrier and a toner, and an image-forming method making use of this two-component type developer.
2. Related Background Art
As electrophotography, various methods are disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth. In these methods, copies or prints are obtained by forming an electrostatic latent image on a photoconductive layer upon irradiation of a light image corresponding to an original, subsequently causing a toner to be attracted onto the electrostatic latent image (electrostatically charged image) to develop the electrostatic latent image to form a toner image, and transferring the toner image to a transfer medium such as paper as occasion calls, followed by fixing by heat, pressure, heat and pressure, or solvent vapor.
In recent years, the advancement of computers and multimedia has brought about a demand for means of outputting more highly minute full-color images, which is made in a wide field including offices and up to homes. Heavy-duty users require a running performance high enough to cause any lowering of image quality even in many-sheet copying or printing. In small offices and homes, users demand to enjoy high-quality images and also make apparatus smaller in size from the viewpoints of saving of space and saving of energy, to reuse waste toner or use a waste-tonerless (cleanerless) system and to make fixing temperature lower. To achieve these objects, various studies are being made from individual viewpoints.
In electrophotography, the step of developing the electrostatic latent image is a step where the toner image is formed on an electrostatically charged image by utilizing an electrostatic mutual action between toner particles triboelectrically charged and the electrostatically charged image. Among developers for developing electrostatic latent images by the use of toner, one-component type developers making use of a magnetic toner comprised of a resin having a magnetic material dispersed therein and two-component type developers formed of a blend of a non-magnetic toner and a magnetic carrier are available. In particular, the latter is preferably used in full-color image-forming apparatus such as full-color copying machines and full-color printers, for which a high image quality is required.
Carriers used in the two-component type developers are known to include iron powder carriers, ferrite carriers, and magnetic-material-dispersed resin carriers comprised of a binder resin having magnetic fine particles dispersed therein. In the iron powder carriers, the carrier has so low a specific resistance that the electric charges of the electrostatic latent image may leak through the carrier to disorder the electrostatic latent image to cause faulty images.
In the case when the ferrite carriers are used, having a relatively high specific resistance, the leak of electric charges of the electrostatic latent image through the carrier can not be prevented in some cases, especially in developing methods where alternating electric fields are applied. Since such carriers have a great saturation magnetization, the magnetic brush may become rigid to cause brush marks in toner images.
To solve such problems, the magnetic-material-dispersed resin carriers are proposed which are comprised of a binder resin having magnetic fine particles dispersed therein. Compared with the ferrite carriers, such magnetic-material-dispersed resin carriers have a relatively high specific resistance and also have a small saturation magnetization and a small true specific gravity, and hence the magnetic brush of the carrier does not become rigid and good toner images free of any brush marks can be formed.
Conventional two-component type developers also have a nature of causing what is called xe2x80x9ccarrier-spentxe2x80x9d, in which a mechanical impact such as impact between particles themselves and between particles and developing machine components, or heat generation caused by such impact, makes part of toner particles adhere physically to the surfaces of carrier particles to form films. Once this has occurred, films ascribable to toner components come to accumulate gradually on the carrier particle surfaces, so that the triboelectric charging that takes place between carrier particles and toner particles may be displaced by the triboelectric charging that takes place between toner particles themselves, and the triboelectric charging performance of the whole developer may deteriorate to further cause what is called xe2x80x9cbackground fogxe2x80x9d, a phenomenon in which much toner adheres to the background areas of copied images, resulting in a low copy image quality. Moreover, there is a problem that, where the formation of films ascribable to toner components on the carrier particle surfaces has become remarkable, the whole developer must be changed for new one and this leads to a cost increase.
On the other hand, the magnetic-material-dispersed resin carriers mentioned above have a small saturation magnetization and also have a small true specific gravity, and hence they are advantageous against such carrier-spent and have an advantage that a small true specific gravity enables developing assemblies light-weight.
The magnetic-material-dispersed resin carriers also have less configurational strain in particles and can relatively easily be made to have spherical shape having a high particle strength. Hence, they have superior fluidity and also enable wide control of particle size. Accordingly, they are expected to be applicable in high-speed copying machines and high-speed laser beam printers.
In such magnetic-material-dispersed resin carriers, however, the magnetic material may come off from carrier particles, thus they have room for further improvement in service durability (running performance).
There is a further problem. In the toner used in two-component type developers, it is preferable to add inorganic fine particles as an external additive to the surface of toner particles, for the purpose of improving charging performance, fluidity and transfer performance. When, however, the magnetic-material-dispersed resin carrier is used in combination with a toner to which such inorganic fine particles have externally been added, what is called xe2x80x9cexternal-additive adhesionxe2x80x9d may occur, which is a phenomenon in which the hills of unevenness (hills and dales) present at the particle surfaces of the magnetic-material-dispersed resin carrier scrape the inorganic fine particles present on the surfaces of toner particles and the inorganic fine particles thus scraped adhere to the dales. As the result, the charge-providing ability the carrier should imparts to the toner may lower and in addition the charging performance, fluidity and transfer performance of the toner itself may be damaged to cause faulty images.
The external additive having adhered to the dales of particles surfaces of the magnetic-material-dispersed resin carrier behaves like the toner at the time of development, and participates the development on the photosensitive member by the action of an alternating electric field formed at the developing zone upon application of an alternating bias voltage to a developer-carrying member developing sleeve. Thereafter, it is further transferred onto a recording medium and then fixed in a fixing assembly, where part thereof remains on the photosensitive member without being transferred, and is removed by means of a cleaning member and collected in a cleaning assembly. Hence, excessive inorganic fine particles are fed to the fixing assembly and cleaning assembly, so that such inorganic fine particles may shorten the lifetime of the fixing assembly and cleaning assembly and may greatly affect even the main body of the image-forming apparatus.
To solve these problems, a method of coating the particle surfaces of the magnetic-material-dispersed resin carrier with a resin to obtain a resin-coated carrier may preferably be used. Also, since the charging performance of the toner can be controlled with ease by the charging properties of the coating resin, the coating resin may be selected so as to impart the desired electric charges to the toner.
Meanwhile, compared with the ferrite carriers, the magnetic-material-dispersed resin carriers have so much fine unevenness on their particle surfaces and so many fine pores in their particle interiors that it tend to have a high water adsorption. Especially in the case of a polymerization type carrier obtained by polymerization carried out in an aqueous medium, which enables easy formation of spherical particles, monomers to be polymerized have a high affinity for water and come into contact with the water in the course of formation. Hence, such a polymerization carrier tends to have a larger water adsorption than any pulverization type magnetic carriers or any magnetic-material-dispersed resin carriers obtained by polymerization in a hydrophobic solvent, and may cause fog and toner scatter because of an extreme lowering of charging performance, especially in an environment of high temperature and high humidity.
To solve such a problem, Japanese Patent Application Laid-open No. 09-127736 discloses a proposal that the water adsorption in high-temperature and high-humidity environment is controlled to obtain images having a good gradation and a high image density especially in the high-temperature and high-humidity environment.
However, this problem has not been solved in the resin-coated carrier, which is effective for carrier-spent resistance and impact resistance and is advantageous also against toner-spent.
Moreover, compared with the ferrite carriers, the magnetic-material-dispersed resin carriers tend to have a higher resistance, and may come to have a much higher resistance when the particle surfaces are coated with resin in order to solve the above problem. Where images are formed using such a carrier in a low-temperature and low-humidity environment, images with sharp edges can be obtained, but on the other hand a problem may occur such that an image with a large area has a very low image density at its central part. Also, the carrier may be charged with substance other than the toner, such as walls inside a developing assembly to make non-uniform the charge quantity of toner that is to be originally imparted. Such a problem may also occur when carriers are manufactured, leaving a problem on safe manufacture.
As discussed above, taking account of the performances required for carriers, the conventional carriers still remain to have problems to be solved, and are sought to be further improved.
In particular, in the magnetic-material-dispersed resin carriers comprised of a binder resin having magnetic fine particles dispersed therein, it is long awaited to provide a carrier more improved in;
(1) running performance;
(2) environmental properties;
(3) carrier-spent resistance, inclusive of resistance to external-additive adhesion;
(4) charge-providing performance to toner;
(5) development contribution;
(6) prevention of carrier adhesion to the photosensitive member surface; and
(7) prevention of toner deterioration.
An object of the present invention is to provide a magnetic-material-dispersed resin carrier having solved the above problems, a two-component type developer having such a magnetic-material-dispersed resin carrier and a toner, and an image-forming method making use of this two-component type developer.
Another object of the present invention is to provide a magnetic-material-dispersed resin carrier having a superior running performance, a two-component type developer having such a magnetic-material-dispersed resin carrier and a toner, and an image-forming method making use of this two-component type developer.
Still another object of the present invention is to provide a magnetic-material-dispersed resin carrier having superior environmental properties, a two-component type developer having such a magnetic-material-dispersed resin carrier and a toner, and an image-forming method making use of this two-component type developer.
A further object of the present invention is to provide a magnetic-material-dispersed resin carrier having superior carrier-spent resistance, inclusive of resistance to external-additive adhesion, a two-component type developer having such a magnetic-material-dispersed resin carrier and a toner, and an image-forming method making use of this two-component type developer.
A still further object of the present invention is to provide a magnetic-material-dispersed resin carrier having a superior charge-providing performance to toner, a two-component type developer having such a magnetic-material-dispersed resin carrier and a toner, and an image-forming method making use of this two-component type developer.
A still further object of the present invention is to provide a magnetic-material-dispersed resin carrier that can contribute to the improvement of developing performance, a two-component type developer having such a magnetic-material-dispersed resin carrier and a toner, and an image-forming method making use of this two-component type developer.
A still further object of the present invention is to provide a magnetic-material-dispersed resin carrier effective for the prevention of carrier adhesion to the photosensitive member surface, a two-component type developer having such a magnetic-material-dispersed resin carrier and a toner, and an image-forming method making use of this two-component type developer.
A still further object of the present invention is to provide a magnetic-material-dispersed resin carrier effective for the prevention of toner deterioration, a two-component type developer having such a magnetic-material-dispersed resin carrier and a toner, and an image-forming method making use of this two-component type developer.
To achieve the above objects, the present invention provides a carrier for use in electrophotography, comprising carrier cores comprising a binder resin and metal compound particles dispersed therein, and a coating resin for coating the surfaces of the carrier cores, wherein;
water adsorption TH2O-H (% by weight) after the carrier has been left in an environment of 30xc2x0 C./80% RH, water adsorption TH2O-L (% by weight) after the carrier has been left in an environment of 23xc2x0 C./5% RH, and particle surface area Sm (cm2/g) of the carrier satisfy the following relationship.
6.00xc3x9710xe2x88x926xe2x89xa6TH2O-H/(100xc3x97Sm)xe2x89xa61.50xc3x9710xe2x88x925,
and
1.00xc3x9710xe2x88x926xe2x89xa6TH2O-L/(100xc3x97Sm)xe2x89xa65.50xc3x9710xe2x88x926.
The present invention also provides a two-component type developer comprising a toner and a carrier, wherein;
the toner contains at least a binder resin and a colorant and has a weight-average particle diameter of from 3 xcexcm to 10 xcexcm;
the carrier has carrier cores comprising a binder resin and metal compound particles dispersed therein, and a coating resin for coating the surfaces of the carrier cores; and
water adsorption TH2O-H (% by weight) after the carrier has been left in an environment of 30xc2x0 C./80% RH, water adsorption TH2O-L (% by weight) after the carrier has been left in an environment of 23xc2x0 C./5% RH, and particle surface area Sm (cm2/g) of the carrier satisfy the following relationship.
6.00xc3x9710xe2x88x926xe2x89xa6TH2O-H/(100xc3x97Sm)xe2x89xa61.50xc3x9710xe2x88x926,
and
1.00xc3x9710xe2x88x926xe2x89xa6TH2O-L/(100xc3x97Sm)xe2x89xa65.50xc3x9710xe2x88x926.
The present invention still also provides an image-forming method comprising the steps of;
charging an electrostatic latent image bearing member electrostatically by a charging means;
exposing the electrostatic latent image bearing member thus charged, to form an electrostatic latent image on the electrostatic latent image bearing member;
developing the electrostatic latent image by a developing means having a two-component type developer, to form a toner image on the electrostatic latent image bearing member;
transferring the toner image formed on the electrostatic latent image bearing member, to a transfer medium via, or not via, an intermediate transfer member; and
fixing the toner image transferred onto the transfer medium, by a heat-and-pressure fixing means;
wherein;
the two-component type developer has at least a toner and a magnetic-material-dispersed resin carrier; the toner contains at least a binder resin and a colorant and has a weight-average particle diameter of from 3 xcexcm to 10 xcexcm;
the magnetic-material-dispersed resin carrier has carrier cores comprising a binder resin and metal compound particles dispersed therein, and a coating resin for coating the surfaces of the carrier cores; and
water adsorption TH2O-H (% by weight) after the carrier has been left in an environment of 30xc2x0 C./80% RH, water adsorption TH2O-L (% by weight) after the carrier has been left in an environment of 23xc2x0 C./5% RH, and particle surface area Sm (cm2/g) of the carrier satisfy the following relationship.
6.00xc3x9710xe2x88x926xe2x89xa6TH2O-H/(100xc3x97Sm)xe2x89xa61.50xc3x9710xe2x88x925,
and
1.00xc3x9710xe2x88x926xe2x89xa6TH2O-L/(100xc3x97Sm)xe2x89xa65.50xc3x9710xe2x88x926.